Switches, in particular in the form of low-voltage circuit breakers, are known and are used to interrupt an alternating electric current flowing through a conductor. They are used to distribute the current in electrical switchgear and to protect the latter. The switches have switching contacts which are separated from one another by means of a switching mechanism in order to open the switch. Current transformers in the form of an air-cored coil (Rogowski coil) arranged at the conductor act as measurement transformers and generate an analog measurement voltage corresponding to the alternating current, the conductor acting as primary winding. In the case of sustained overloading or in the case of a short circuit, the switch is tripped using the measurement voltage in a manner dependent on parameters and characteristics. For this purpose the measurement voltage is applied to an electronic trip unit which compares the former with a predefined threshold value, the exceedance of which causes the switching contacts to be separated.
In order to supply power (inherent supply) to the trip unit, power transformers (usually iron-cored transformers), which are likewise arranged at the conductor, are used. The reliability of the power supply depends significantly on the power transformer. Circuit interruptions or contact problems in the power transformers can lead to a total failure of the protective function of the switch.
A rectifier circuit is connected to the power transformer, said rectifier circuit charging a capacitor which feeds the power supply, wherein a switching device short-circuits the rectifier circuit in each case when a predefined capacitor voltage (nominal voltage) is reached in order to keep said voltage approximately constant.
In a three-phase power supply system, the current from the power transformers is rectified, for example by means of a three-phase bridge rectifier extended by the neutral conductor input. In this case the circuit is extended by a neutral conductor star point. A capacitor is used here as an energy store. A switching device, for example in the form of a power transistor connected in parallel, short-circuits the power transformer when the capacitor nominal voltage is reached, and thus prevents this from rising further.
In this circuit variant, the current of each individual power transformer cannot be evaluated in a differentiated way for each channel. As a result, it is also not possible to determine which input of the power transformer supplies power and which does not, particularly as compensating currents flow between the power transformers in the three-phase bridge rectifier.
Certain known power supplies have the disadvantage that it is not possible to monitor the power transformers continuously. Any possible damage and, therefore, a malfunction of the switch cannot be detected and signaled, with the result that installation operators cannot immediately exchange the power transformers or the switch.
An overcurrent relay for electrical installations, having a power transformer in the form of a transformer for supplying power to an electronic unit, in particular a trip unit, is already known from DE 690 13 683 T2. In addition to the power transformer, a measurement transformer formed from current-sensing resistors is also present. The power transformer is connected to a rectifier circuit which charges a capacitor to a predefined voltage. Furthermore, the overcurrent relay has a monostable multivibrator to which a signal tapped off from the power transformer is fed via an electronic circuit. An AND gate is connected to the monostable multivibrator and supplies an overload signal in the case of an overload. The overload signal of the AND gate triggers the monostable multivibrator, which causes a transistor switch to turn on in order to trip the overcurrent relay. The monostable multivibrator is used to generate a pulse of sufficient length and thus to ensure in each case that the overcurrent relay is tripped, since the pulse length of the AND gate is not always sufficient for this.